1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to hydraulically actuated die closing units of injection molding machines in which the movable parts of the die closing unit are supported and guided by means of two or more parallel tie rods.
2. Description of the Prior Art
Although there exist several different types of die closing units, when classified in accordance with the mode in which the die closing movement is generated and in accordance with the means by which the injection molding dies are clamped together during injection, virtually all of them employ tie rods, in most cases two or four parallel tie rods, along which the movable parts of the die closing unit are guided relative to the stationary parts, and which also serve to apply the clamping force to the die plates or to hold the stationary die plate against the clamping force which is applied through the movable die plate.
Accordingly, the tie rods of a die closing unit are subjected to cyclically occurring elevated tension stress. On the other hand, the tie rods need to be manufactured and mounted with extreme precision, in order to assure the required precision of die plate guidance and to provide the precise parallel alignment of the die mounting plates which is necessary for the production of injection molded parts of high quality.
While it has heretofore been the common practice to connect the tie rods to the stationary and/or movable die plates of the die closing unit by providing on at least one extremity of the tie rods an externally threaded end portion of reduced diameter, with ring shoulders on the tie rods positioning the plate in question, I have previously suggested a tie rod connection in which the need for such a reduced diameter tie rod end portion is eliminated, and where the diameter of the tie rod attachment bores in the plate, or plates, is the same as the diameter of the guide portion of the tie rods. Such a tie rod connection is disclosed in my copending application, Ser. No. 689,813, filed May 25, 1976, now U.S. Pat. No. 4,021,181.
This earlier application suggests a tie rod connection in which the tie rod reaches through the attachment bore with a tight fit and has a shallow annular abutment groove in a tie rod portion which protrudes beyond the plate or thrust member, so that the groove is located on the outer side of the plate, a split abutment ring being seated in the annular groove in such a away that one axial end face of the abutment ring bears against the outer side of the thrust member, while the other axial end face of the ring bears against a flank of the abutment groove. This abutment configuration is preloaded in the axial direction by means of a clamping cap which encloses the tie rod extremity so as to radially retain the split abutment ring in its groove, while applying an axially inwardly directed preload to the tie rod itself, under the action of clamping bolts which pull the clamping cap towards the thrust member.
The advantages which flow from this earlier solution relate to the elimination of the stress concentration which is unavoidably associated with the ring shoulder at the transition between the reduced diameter tie rod end portion and its full diameter. It is a well-known fact that, due to the stress concentration which is associated with such a ring shoulder, a tie rod of stepped diameter configuration is considerably less resistant than a tie rod whose diameter is equal to the diameter of the smaller end portion, but which is continuous and without a shoulder.
An additional important advantage of my earlier solution relates to the fact that a tie rod with a clamping shoulder and counter-acting clamping nut on the opposite side of the thrust member needs to have a preload applied to the clamping nut which is higher than the peak cyclical tension stress on the tie rod, in order to preclude the possibility that the clamping shoulder temporarily disengages itself from the thrust member. The arrangement of an abutment groove and split abutment ring on the outside of the thrust member eliminates this need, although a lesser preload is still necessary.
This earlier solution also enhances the structural rigidity and dimensional accuracy of the frame which is formed by the tie rods and the attached plate, or plates. Another important advantage flows from the fact that this tie rod connection allows for the tie rods to be retracted through their attachment bores in both axial directions, following removal of the clamping cap and of the split abutment ring from at least one tie rod extremity.
A complete die closing unit taking advantage of this tie rod connection is disclosed in my copending application, Ser. No. 640,531, filed Dec. 15, 1975, now U.S. Pat. No. 4,025,264. There, it can be seen that the improved tie rod connection makes it possible to much more efficiently utilize the die mounting space which is available between the four tie rods of a die closing unit, if at least one of the tie rods is retractable for the mounting of the injection molding die on the die carrying plates. The retractability of the tie rods offer the further advantage that the tie rods themselves can be used as centering members for the die halves.
In spite of these important advantages, the earlier solution has certain inherent limitations. One of these limitations relates to the fact that the abutment groove in the tie rod, although shallow, represents itself a stepped diameter configuration, where the tie rod has a weakened cross section. Associated with it is a certain degree of stress concentration, although considerbly less pronounced than that of the prior art ring shoulder which engages the thrust member from the opposite side of the clamping nut.
Another limitation which is inherent in my earlier solution relates to the fact that the outer flanks of the abutment grooves have to be located with extreme precision in the axial sense, since it is these flanks, in conjunction with the axial width of the split abutment rings, which determine the position of the thrust member to which the tie rods are attached. Even precisely identically machined the tie rods may present a problem in this regard: tie rods of initially identical length may undergo different degrees of elongation during heat treatment, dependng upon the specific alloy composition of the steel used, which itself may vary within certain alloy composition tolerances. It thus may become necessary to separately regrind the width of the split abutment rings, in order to compensate for axial alignment deviations between the abutment groove flanks of the several tie rods. If left without compensation, these alignment deviations will reflect themselves in corresponding alignment deviations of the attached thrust member or plate, leading to corresponding alignment errors between the die halves on the die carrier plates, and at the same time imposing bending moments on the tie rods which may create friction inducing tie rod distortions.